1. Field of the Invention
The present invention relates to the measurement and correction of telecentricity in a lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, light is directed to a patterning device, which can be referred to as a mask, a reticule, an array of individually programmable or controllable elements (maskless), or the like. The patterning device can be used to generate a circuit pattern corresponding to an individual layer of an IC, flat panel display, or other device. This pattern can be transferred onto all or part of the substrate (e.g., a glass plate, a wafer, etc.), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. The imaging can include the processing of light through a projection system, which can include optical components such as mirrors, lenses, beam splitters, and the like. Other components or devices can exist in a lithographic apparatus that can also contain optical components, such as a multi-field relay (MFR), which contains optical components to divide a radiation beam into a number of individual beams prior to patterning.
Lithography requires the projection of patterns onto a substrate with a high degree of accuracy. However, relative movement between the optical components of the lithographic apparatus may affect the alignment of the apparatus and induce telecentricity effects that degrade the accuracy of the projected pattern. In particular, the projection system may be especially sensitive to telecentricity induced by relative movement between the critical optical components of the illumination system.
In general, maskless lithographic apparatus are more sensitive to telecentricity than are mask-based lithographic apparatus. The increased sensitivity to telecentricity experienced by many generally-known maskless lithographic apparatus results substantially from the lens magnification used within these apparatus. The lens magnification employed by many maskless lithographic apparatus may be two orders of magnitude larger than the lens magnification in comparable mask-based lithographic apparatus. As sensitivity to telecentricity effects scales with lens magnification, the telecentricity tolerances required by common maskless lithographic apparatus may be two orders of magnitude tighter than those required by comparable mask-based lithographic apparatus. A similar telecentricity tolerance would be required in a mask-based lithographic apparatus that employs a lens magnification of magnitude similar to that commonly employed in maskless lithographic apparatus.
In an effort to reduce relative movement, the critical optical components of maskless lithographic apparatus are generally held stable with respect to each other on a number of isolated structures. Further, in order to ensure that projection is achieved at a high degree of accuracy, the lithographic apparatus may perform various calibration measurements, and in some instances, the lithographic apparatus may be adjusted in response to these measurements.
Unfortunately, it is difficult to completely eliminate relative movement between the optical components in lithographic apparatus. Further, existing calibration techniques are generally insufficient to perform real-time measurement and correction of telecentricity. These techniques often use actinic light to measure telecentricity, and the exposure of the substrate must be halted during measurement and subsequent alignment. As such, existing techniques to measure induced telecentricity in lithographic apparatus make inefficient use of actinic light and reduce the throughput of the lithographic apparatus.